Stepping motors are available in which the motor rotates a predetermined amount in response to a single input pulse. The rate of motor rotation is therefore proportional to the repitition rate of the input pulses. Typically, the input pulses are metered or counted, such that a pulse group with a given number of pulses will rotate the motor a predetermined amount. Such arrangements are useful in positioning systems in a wide variety of fields.
Since the stepping motor is limited in the rate at which it can properly respond to input pulses, the input pulse repitition rate and therefore the rate at which the motor can be stepped is likewise limited. However, while the stepping motor has a first pulse repitition rate limit as the limiting rate of input pulses which can properly start if from rest, the motor can be induced to step at a higher velocity than that associated with this rate limit, if the rate of input pulses can be gradually increased. Like remarks apply to deceleration as well. Accordingly, acceleration/deceleration control circuits have been provided in the prior art, such as that disclosed in U.S. Pat. No. 3,579,279.
As disclosed, a source of constant rate input pulses are provided to a control circuit which includes a plural stage up/down counter. A variable rate pulse source is controlled in rate by the output of a digital to analog converter connected to the output of the counter. The pulse source output is supplied as an input to the stepping motor and is also fed back and coupled to the up/down counter to count the counter down. This arrangement is effective to alleviate to some extent the problems caused by the limited rate response of the stepping motor. As a group of constant rate pulses is first applied to the up/down counter, the counter begins counting up. In the initial stages of the up counting operation, the output of the digital/analog converter begins to increase from zero at a constant rate. As the output of the digital/analog converter first moves above zero, the variable rate pulse source begins emitting pulses, and as the voltae increases, so then does the pulse rate. However, as soon as the pulse source begins emitting pulses, it starts to count down the up/down counter so that the rate of increase in the count of the counter begins to decrease. In this fashion, the pulse source rate change begins slowing down until an equilibrium condition is reached at which the pulse source rate matches the input pulse rate. This is a stable condition and will continue for so long as the input pulses are present. When the input pulses disappear, however, the counter begins counting down rapidly. As the count in the counter changes in the downward direction, of course, the rate of the pulse source also changes, thus decreasing the rate at which the pulse rate changes. One result of the control circuit is that it emits the identical number of pulses as are applied to it and thus the stepping motor is moved through the desired angle. Also, the abrupt changes in pulse rate are "smoothed" to some extent, allowing a higher input pulse rate than would be the case without the control circuit.
However, the output rate of the pulse source, i.e., the pulses actually applied to the stepping motor, still exhibit abrupt changes especially at the initial acceleration and at initial deceleration times. In addition, the exponential fall also results in undesirably long time for stopping. Thus, while the prior art control circuits do allow the use of higher input pulse rates, there is still a desire to improve the response of the overall arrangement such that even higher input rates can be employed.
It is therefore one object of the present invention to provide an accelerationdeceleration control for a stepping motor which "smooths" the rate changes of the prior art arrangements. It is another object of the present invention to improve the prior art acceleration/deceleration control circuits of stepping motors to allow higher input pulse rates to be employed without causing the stepping motor to fall out of step.